1. Field of the invention
The present invention relates to glass slumping processes for shaping glass sheets and, more particularly, to a glass slumping process for shaping a single sheet of glass into a shape for use as a lamp shade.
2. Description of the Related Art
Various processes have been proposed for shaping glass sheets to impart a bend or curvature thereto. Most glass shaping processes are of the "dropout" type. In dropout processes, a sheet of glass is secured or supported along its perimetrical edge over a concave cavity of a mold. The glass is heated so that it softens, causing it to drop or sag into the cavity due to its own weight. The glass sheet thusly assumes a concave shape conforming to the concave cavity of the mold. Illustrative dropout processes for shaping glass sheets are disclosed in U.S. Pat. No. 497,574 to Heckert, U.S. Pat. No. 544,248 to Cutler, U.S. Pat. No. 643,223 to Dhe et al, U.S. Pat. No. 787,438 to Cutler, U.S. Pat. No. 1,161,998 to Titus et al, U.S. Pat. No. 3,510,284 to McMaster et al, U.S. Pat. No. 3,560,183 to Stilley et al, U.S. Pat. No. 4,119,424 to Comperatore, U.S. Pat. No. 4,229,201 to Comperatore et al and U.S. Pat. No. 5,573,566 to Anderberg et al.
A modification of a dropout process wherein a glass sheet drops into a concave cavity of a mold having a plurality of ribs extending radially above a concave surface of the cavity is shown in U.S. Pat. No. 2,503,653 to Boyles et al. In the Boyles et al process, the glass sags a greater amount between the ribs to assume a scalloped or fluted shape for use as a lamp shade.
Vacuum forming processes have also been used to shape glass sheets that have been softened by heating. U.S. Pat. No. 4,349,374 to Rupp is representative of a vacuum forming process. In addition, vacuum forming has been used in combination with dropout processes to promote sagging of the glass sheets into cavities of the molds as exemplified by Cutler '248 and '438, McMaster and Anderberg et al. Numerous disadvantages and drawbacks are associated with dropout processes for shaping glass sheets. The perimetrical edges of the glass sheets secured to the molds are prevented from dropping or sagging into the cavities of the molds. This results in formation of protruding, perimetrical flanges in the glass sheets after shaping, such flanges circumscribing or surrounding portions of the glass sheets that have been allowed to drop or sag into the cavities. Depending on the intended use for the shaped glass sheets, such flanges may be undesirable and require removal. Removal of the flanges adds additional cost, labor and time to the glass shaping processes. Another drawback of dropout processes is that the glass sheets tend to be undesirably stretched when they drop into the molds. Additionally, although dropout processes are effective for relatively thick (e.g. 1/2-1 inch thick) glass sheets, such processes are generally unsuitable for relatively thin (e.g. 1/4 inch thick or less) glass sheets. Most dropout processes involve heating the glass sheets relatively rapidly, which is detrimental to relatively thin glass sheets and/or "art" glass sheets. Many dropout processes also involve multiple heating cycles (firings), movement of the glass sheets during heating, repositioning of the glass sheets in or on the molds and/or transfers of the glass sheets between different molds, thusly adding undesirably to the complexity, cost, time and labor for the glass shaping processes. Furthermore, dropout processes typically involve contacting the glass sheets with chemicals, gases or extraneous mechanical components, which is highly undesirable when shaping art glass sheets.
Vacuum forming processes for shaping glass sheets possess many of the drawbacks and disadvantages associated with dropout processes. In addition, the glass sheets may crack or otherwise be impaired due to inconsistencies or lack of control in the applied vacuum. Moreover, the appearance of the glass may be undesirably altered due to application of the vacuum, which is a significant drawback when aesthetics are important, such as when using art glass.
It has also been proposed to slump heated glass sheets in a convex manner, rather than the concave manner associated with dropout processes. Exemplary "slumping" processes are represented by Rupp and by U.S. Pat. No. 2,218,654 to Paddock. In Rupp, the heated glass sheet does not naturally slump due to gravity but, rather, is forced to slump via an applied vacuum. Accordingly, the process disclosed by Rupp is essentially a vacuum forming process and is associated with the various disadvantages and drawbacks of vacuum forming processes discussed above. In addition, the process disclosed by Rupp involves slumping only a portion of the glass sheet, which must initially be provided with concave and convex surfaces. The process disclosed by Paddock involves supporting a transverse middle section of a glass sheet upon spaced parallel rollers defining parallel lines of transverse support for the glass sheet. After being heated, two opposing ends or sides of the glass sheet slump down while the middle section remains supported on the rollers to form an initial bend, but not a dome shape. The glass sheet is then placed in a concave mold to obtain a final, greater bend by heating the glass sheet to cause it to drop into a concave cavity of the mold. The process of Paddock has many of the same disadvantages and drawbacks discussed above, particularly since the Paddock process is basically a dropout process and requires two heating cycles or firings.
In the area of art glass shaping, conventional glass shaping processes have proved to be inadequate. Art glass, such as that made by Youghiogheny Glass of Youghiogheny, Pennsylvania, possesses unique compositlonal and behavioral characteristics different than those of conventional plate, crown, kitchen, window or sheet glass to which most of the processes discussed above are directed. Such art glass typically contains visible colorations, striations, stipples, mottles or other patterns, iridescence, opacity, brilliance, luster and/or radiance created by particular crystallization and/or precise glass chemistry and control of heat differentials when the glass is being formed. The "Stipple" line of art glass of Youghiogheny Glass is representative of an art glass that has a unique molecular structure. Due to the unique characteristics and behavior thereof, such art glass is extremely difficult to shape using conventional glass shaping apparatus and processes. For instance, the amount of time used in conventional glass shaping processes to heat glass sheets to a maximum temperature and the rate at which the temperature is increased during such time have been found by the present applicants to be inappropriate for shaping art glass and, in particular, for shaping a single sheet of art glass into a configuration for use as a lamp shade. Other parameters of conventional glass shaping processes, such as the particular maximum temperature, the duration of the maximum temperature, the rate at which the glass is cooled and the temperature at which the shaped glass is removed from the kiln, have also been found by the present applicants to be inapplicable in various respects to shaping art glass. Since art glass is highly sensitive, even small variations in process parameters may produce detrimental results. Furthermore, in order to maintain the purity and appearance of art glass, it is desirable to avoid exposure of the art glass to chemicals, gases or other foreign substances. In addition, it is desirable to greatly minimize contact of the art glass with the mold being used as well as any other extraneous mechanical components or structure.
Accordingly, the need exists for a slumping process for glass sheets, particularly sheets of art glass, wherein the glass sheets, upon being heated, are allowed to fall naturally over a convex mold to obtain a final shape in a single heating cycle or firing. The need further exists for a glass slumping process applicable to relatively thin sheets of art glass. There is also a need for a slumping process wherein natural slumping of glass sheets upon being heated is controlled or guided by protrusions or arms on a convex mold. An unfulfilled need also exists for a glass slumping process having process parameters specifically adapted for art glass, particularly the art glass of Youghiogheny Glass of Youghiogheny, Pa.